Chemically reactive low-pressure plasma processes have become standard in the coating and etching of semiconductor wafers. Apart from the use of such plasma for the machining of flat substrates, plasma processes can be used for the treatment of objects of complex shapes, for example, molded plastic products, fiber bundles and materials in the form of webs or strips. There are a number of processes and apparatuses available which serve for the plasma treatment of substrates of different shapes, purposes and materials.
DE 31 17 257 C2, for example, describes an apparatus for plasma deposition of thin films in which the microwave power is outputted from a rectangular waveguide through a coupling window of a dielectric material into a cylindrical resonance chamber which simultaneously serves as a plasma chamber. Apparatus of this type and those which use air-core coils or composites of permanent magnets around the plasma chamber for the purpose of generating the electron-cyclotron-resonance, produce a directed plasma beam which can be employed, for example, for disk-shaped substrates like the wafers described previously. Large substrates, however, can only be treated by movement of them in the plasma beam or with the simultaneous use of a plurality of such plasma sources.
In DE 40 38 091 A1, an apparatus is described for generating a controllable microwave field which enables the ignition of a plasma which is homogeneous over a relatively long path. This is achieved by outputting the microwave power through a row of preferably inductive coupling antennae from a rectangular or cylindrical resonator.
On the vacuum side, the antennae are in contact with the plasma and as a consequence, a contamination of the substrate with the antenna material cannot be avoided. While with this earlier apparatus the machinery of large-area substrates is possible, the apparatus does not solve the problem of treating with plasma a large-volume substrate from all sides.
The French patent document FR-A-2 112 733 describes an apparatus for uniform microwave power coupling into a cylindrical volume and in which a standing wave is generated in a cylinder formed with a slit antenna. The wave pattern can meander. The connection of a microwave generator with the antenna here can be realized only with a coaxial cable.
This limits the possibility of upscaling this type of microwave excitation of a plasma because, with such a cable, transferable power over long periods of time with stability cannot exceed several hundred watts.
In the European patent EP 0 209 469 A1, an apparatus for producing an electron-cyclotron-resonance plasma is described in which the inputting of the microwave power is effected by a multiplicity of antennas which are disposed on the wall of the plasma chamber. The plasma chamber is surrounded by permanent magnets which, in the vicinity of the antennas, produce electron-cyclotron-resonance regions. As a consequence of the ion sputtering, the antennas directly in the plasma constitute a source of metallic contamination. To avoid such contamination EP 0 402 282 A2 describes an apparatus in which shielding plates are provided between the antennas and the plasma and upon which the substrate material can be captured. The disadvantageous effect of sputtering or atomization of the antenna materials is thereby reduced, although it is not completely eliminated.
Both of these earlier systems have the common drawback with respect to the nature of the distribution of the microwave power to the antennas. The apparatus for power splitting is comprised of a waveguide and corresponding connecting couplers and cannot guarantee a uniform coupling of the microwave power to all of the antennas and thus the formation of a cylindrical symmetrical plasma cannot be ensured.
A direct inputting of microwave power from a rectangular waveguide into an annular resonator is disclosed in EP 0 398 832 A1. The dielectric inner wall of the resonator here simultaneously fulfills the function of a plasma chamber wall through which the microwave power is outputted to the plasma. The flat sides of the annular resonator are provided as shunt slides which enable adjustment of the optimum resonance conditions. However, since here the plasma chamber has the dielectric walls, the dimensions of a substrate to be treated in the chamber is limited. Furthermore, a complex and expensive mechanical arrangement is required for tuning of the resonator.